1. Technical Field of the Invention
The present invention relates in general to paving machines for paving a path of limited width. More specifically, the present invention relates to an improved screed plate adapted for attaching to a screed of a paving machine.
2. Description of the Related Art
Most asphalt pavers are large and intricate machines comprising a tractor unit and a screed unit. The screed unit operates based on the principle of the self-leveling and floating screed. These devices are well known and are used for leveling and consolidating paving materials such as asphalt, concrete and the like. The screed unit includes a replaceable screed plate, constructed of suitable steel, to spread paving material (asphalt, aggregate, concrete, petroleum bitumen and the like) to a proper thickness, and to create a smooth surface after the paving material is applied to the roadway during road construction. The screed also provides the initial compaction of paving materials. The screed plate further must have the capacity to conform to the desired profile of the pavement (e.g. a crowned road). These non-level profiles are required to facilitate water runoff, especially during rainstorms. While a simple crowned profile may suffice in many paving scenarios, urban paving poses additional complications and problems as intersections generally require changes in the simple crowned profile so as to allow for multi-directional rainfall runoff. In order to meet these requirements, a screed plate must bend and deflect to a shape that meets the applicable specifications.
FIG. 1 is a perspective view of a prior art paving machine 10 with a screed unit 12 attached thereto. The conventional paver operates based on the principle of the self-leveling, floating screed. Here, the screed unit 12 is a free-floating unit that strikes off, compacts, and flattens raw material into a mat surface as the screed 12 is pulled forward by the paving machine 10. The screed unit 12 is attached to the paving machine 10 at only two points, each called a pull point or tow point, one on each side of the paver. The screed unit 12 has two long pull arms to make this connection to the pull point and in operation, pivot arms 14 are disposed between the pull arms and the pull points and provide no vertical support for the screed unit 12. As the paving machine 10 pulls the screed unit 12 into the freshly laid material, it floats on the mix of paving material, moving up or down and naturally seeking the path where the forces acting on the screed unit 12 are in equilibrium and the path of its flat bottom is approximately parallel to the direction of operation of the paver.
FIG. 2 is a perspective view of another prior art screed assembly of the paving machine 20. The screed assembly 20 includes a vibrator 21, which causes the paving material to feed more uniformly under the screed assembly 20 and provide some initial compaction to the freshly laid material. The screed assembly 20 further includes a screed heater 23, which prevents the paving material mixtures from sticking to the screed plate 24. The optional attachments such as cutoff shoes/screed extensions 25 may be attached to the screed assembly 20 to vary the paver width.
Various screed plates have been devised for attaching to the screed unit of a paving machine. U.S. Pat. No. 5,366,320 issued to Hanlon on Nov. 22, 1994 provides an improved screed for leveling abrasive paving material on a road surface. The improved screed is highly abrasion-resistant and loses much less heat during shutdown periods than steel screeds because it is formed of a composite that includes a chromium-carbide alloy. The composite is formed of a relatively soft metal that is completely fused to the chromium-carbide alloy. The part of the composite that comes in direct contact with the abrasive paving material is made of highly abrasion-resistant chromium-carbide alloy, which has a Brinell hardness in the range 550 to 600 and a low coefficient of friction. Additionally, the low thermal conductivity of the chromium-carbide alloy permits it to retain more heat during shutdown. The relatively soft metal portion of the composite, which may be low-carbon steel, provides an easy and reliable surface for affixing standard attachment components, such as bolts, to the improved screed, providing an effective means for mounting the improved screed to the paving machine. A curved leading edge of the screed prevents the paving material from welling up on to the relatively soft upper surface of the improved screed. The curved leading edge is formed of the same composite as the rest of the screed. A shortcoming of the improved screed is that although the metal component forming the bottom surface of the screed is extremely hard and durable, in the Brinell range of 550-600, the top surface of the screed is made of softer metal, in the Brinell range of 150-200. Thus, the top surface of the screed is easily subject to maximum wear and tear from the harsh mechanical forces involved.
U.S. Pat Application. No. 20060045624 to Nelson and published on Mar. 2, 2006 provides a screed heating arrangement for a screed assembly that is towed behind a paving machine. The screed heating arrangement includes at least one electric heater bonded to an upper surface of a screed plate. The screed is made up of hardened steel that is actually stronger than the frame assembly that attaches screed to the paving machine. This can lead to warping of the frame assembly.
Other conventional paving machines include a screed plate having a bottom surface made of softer steel. There, the bottom surface that is dragged along the mat will lose smoothness, and rapid and excessive wear will ultimately lead to early replacement of the screed plate. Conventional screed plates typically wear out about 4 to 6 inches forward from the trailing edge. This is due to the standard angle at which the screed contacts the paving material during use, which lifts the front edge of the screed, placing more pressure and wear and tear on the trailing edge.
Typically the conventional soft screed plates cannot be used in the entire work season, since the soft screed plate can bend and conform to assist with crowning. Even though the conventional harder screed plate may last for the entire season, it is typically too hard to bend and conform as described above.
Hence, it can be seen, that there is a need for an improved screed plate with the flexibility of softer steel and durability of harder steel. Further, the improved screed plate is made of high strength steel that will not warp the screed frame assembly as the screed plate is bent to effect proper crowning. Moreover, the improved screed plate would include a tapered groove possessing sufficient depth at a curved leading edge of the screed plate so as to allow improvements in crowning performance by permitting and proper deflection of the hardened steel.
It is thus a first objective of the present invention to provide an improved screed plate with a flexibility of softer steel and durability of harder steel.
It is a second objective of the present invention to provide an improved screed plate of high strength steel that will not warp the screed frame assembly as the screed plate is flexed and inverted to effect proper crowning.
It is a third objective of the present invention to provide an improved screed plate with a tapered groove that possess sufficient depth at a curved leading edge of the screed plate to allow proper deflection of the harder steel.
It is a fourth objective of the present invention to provide an improved screed plate with a tapered groove that changes in depth approximately 0.187 inches over the course of approximately 15 inches of screed width or about 0.0125 inches per inch.
It is a fifth objective of the present invention to provide an improved screed plate with a tapered groove that allows improvements in crowning and deflection of the screed plate to be accomplished even when a relatively hard inflexible material is used for screed plate, such as 500 and 600 Brinell hardness number (BHN) steel.
These and other advantages and features of the present invention are described with specificity so as to make the present invention understandable to one of ordinary skill in the art.